So the weather in England doesn't always make having an outdoor pool a sensible thing. There are some but the term for an outdoor pool is lido.
In Cambridge, they have the Jesus Green Lido located on Jesus Green. It is a 100m long pool, and is very col because of the size of it but great on a hot day! It is usually open from the late May Bank Holiday to the end of September. Other that this pool, I cannot think any other local outdoor pools in the area.